The high data rate transmission is one of the major features of the present-day wireless communication systems. Conventional single-carrier transmission technologies, such as GSM, cannot meet the needs of high data rate. This is because the signal sampling period becomes very short under the high data rate transmission, it is therefore easily affected by the channel delay spread and leads to severe inter-symbol interferences (ISI). From the frequency's point of view, the signal bandwidth of high data rate transmission is large, and in comparison to the coherent bandwidth of the channel, the signal is apparently influenced by the frequency selective fading effect of the channel. Consequently, when using the single-carrier transmission technology to transmit at high data rate, a complicated equalizer must be used to maintain a good performance. For the complexity of the receivers, it is inefficient to transmit high-rate data for using this type of communication system.
In multiple carrier transmission technologies, the high-rate data are distributed to multiple subcarriers, instead of using a single carrier. For each subcarrier, the data rate is not high, therefore the complexity of the equalizer in a receiver can be reduced. However, the multiple-carrier technologies have three major disadvantages that prevent the technologies from wide use. First of all, it takes a plurality of sinusoidal wave generators and multipliers to compute the data on the multiple subcarriers. Secondly, the subcarrier spacing must be large enough to avoid the inter-carrier interference (ICI) among the data on the multiple subcarriers. This leads to a waste of bandwidth. Finally, peak-to-average power ratio (PAPR) is large because the multiple subcarrier transmission signals are the sum of a plurality of subcarrier signals. This will lead to non-linear distortion when passing the power amplifier during the transmission.
Due to the progress of digital signal processing (DSP) and VLSI in recent years, the difficult resulted from computation complexity is improved. Also, Fast Fourier Transform (FFT) being used to implement the multiple subcarrier transmission further reduces the computational load. The use of FFT also makes the subcarriers orthogonal to each other, which leads to the better bandwidth efficiency. Therefore, this kind of multiple subcarrier transmission technology is called the FFT-based OFDM system. The FFT-based OFDM system has improved the two aforementioned disadvantages.
FIG. 1 shows a block diagram of a typical FFT-based OFDM system including an equivalent baseband transmitter and a transmission channel. Data signals d(n) are mapped respectively on N subcarriers, and an inverse FFT (IFFT) is used to implement the OFDM transmission system. The data signals on each individual subcarrier consist of m bits. The value of m depends on the modulation technology used by the signal mapper. For example, if the mapper uses quadrature phase shift keying (QPSK) modulation, the value of m is 2. The signals x(n) at the IFFT output will cyclically prefix a guard interval to form a complete OFDM symbol before the signals are transmitted into the channel. The guard interval includes Ng sample points, and the useful symbol includes N sample points, as shown in FIG. 2.
In addition to the aforementioned non-linear distortion caused by the large PAPR, the use of orthogonality in the FFT-based OFDM technology also increases the sensitivity to the signal synchronization errors. This puts a higher demand on precision of the detection of the carrier frequency offset. As the carrier frequency offset increases, the orthogonality will be damaged and the system performance will rapidly deteriorate.
In addition to the detection of the carrier frequency offset, the signal synchronization includes the estimation of the symbol timing. The imprecision of the symbol timing estimation will lead to the ISI. The ISI introduced in the OFDM system is smaller than that in the single-carrier system. It is because the duration of a useful OFDM symbol (equivalent to the length of an FFT) is much longer than the duration of a symbol in the single-carrier system at the same data transmission rate. Besides, the guard interval is used to alleviate the ISI effects. Therefore, the symbol timing estimation can tolerate even further imprecision.
Symbol timing estimation in the OFDM system is usually based on the structure of the guard interval, and it uses the delay conjugate multiplication and moving average operation to roughly estimate the symbol timing. The roughly estimated symbol timing information can be used to estimate fractional carrier frequency offset. In general, the channel delay spread is much shorter than the guard interval. Therefore, the rough estimation of the symbol timing will not cause severe ISI effect, and have a relatively smaller imprecise effect on the subsequent carrier frequency offset detection. However, if the channel delay spread is close to the length of a guard interval, the ISI effects due to the imprecision of the rough symbol timing estimation is significant. Besides, it will lead to the larger imprecision of the subsequent carrier frequency offset detection.
The OFDM wireless communication systems can be divided into two categories. The first category is the broadcasting systems, including the European specification digital audio broadcasting (DAB) system and digital video broadcasting-terrestrial broadcasting (DVB-T) system. The second category is the packet switch network, including the wireless local area network (WLAN) IEEE802.1a. The broadcasting system continuously transmits the data and allows more time in signal synchronization. The channel delay spread is usually larger in this category of systems. On the other hand, the packet switch network does not transmit the continuous data, and sometimes the data may be even bursty. So it requires the signal synchronization achieved quickly after the packet transmission starts. The channel delay spread is usually smaller in this category of systems.
The U.S. Pat. Nos. 6,192,052 and 6,449,246 disclosed a symbol timing estimation obtained by the time corresponding to the largest amplitude, based on guard interval, delay conjugate multiplication module, and moving average operation. They used a moving average operation whose length is that of the guard interval. It is more suitable for white Gaussian noise channel, but it is not suitable for a multi-path channel with large delay spread. The U.S. Pat. No. 6,192,052 used the maximum correlation (MC) criterion to improve the symbol timing estimation, and the U.S. Pat. No. 6,449,246 used the maximum likelihood (ML) criterion.
The U.S. Pat. Nos. 6,181,714 and 6,205,188 disclosed a symbol timing estimation obtained by the time corresponding to the largest amplitude, based on guard interval, delay conjugate multiplication module, and symbol-by-symbol average operation. This method is suitable for a multi-path channel with larger delay spread to estimate the location of one point in the ISI-free region, but it is not suitable to estimate the location of the entire ISI-free region.
Another Patent application 2003/0026360, proposed by Motorola, disclosed a method for detecting the ISI-free region, based on guard interval, delay conjugate multiplication module, symbol-by-symbol average operation, and edge detection. However, it is more difficult to determine the value of a fixed optimal threshold to separate the ISI-free region, and it needs more computation complexity to implement.